Credit-rating agency Standard & Poor this morning took aim at the world's fourth richest man. S&P lowered its rating on Berkshire Hathaway, the holding company run by billionaire Warren Buffett, to AA from AA+.

The drop in investment-grade rating, S&P says, reflects Berkshire's dependence on its core insurance business for dividend income. While Berkshire's non-insurance companies do provide most of its operating income, only the railroad Burlington Northern also contributes to dividends.

Also at issue is the uncertainty over who will succeed Buffett as CEO. Buffett, 82, would likely dispute that this is an issue. He has chosen a successor and says Berkshire directors are "solidly in agreement" with his choice. Buffett's shareholder letters are regularly closely read by observers hoping for clues as to the heir's identity. Ajit Jain, the man running the aforementioned insurance business, is often speculated as a possible successor.

S&P earlier this month warned it would change how it assess insurance companies, a portentous signal of rating changes to businesses like Berkshire's. (It could later hit other insurance companies like Allstate, Progressive, Travelers Companies and XL Group.) The new method measures sovereign risk, and S&P hasn't been particularly optimistic about the U.S. Hence, the axe fell on Berkshire's debt, and the continued negative outlook on the rating.

Hardly a roundly negative report from S&P, though. S&P finds Berkshire in an admirable position, with a "very strong financial risk profile, built on extremely strong competitive position and very strong capital and earnings."